1. Field of the Invention
The invention relates to light emitting diode (LED) devices, and more particularly to a white light emitting diode device (WLED) and a fabrication method thereof.
2. Description of the Related Art
Light emitting diode (LED) devices are widely used in display products because of their advantages such as brightness, small size, light weight less power consumption and longer lifetime. Additionally, since white emitting diode (WLED) device was developed by Nichia Company, application of WLED device have expanded more and more to illumination product. Compared to incandescent or fluorescent lamps, the advantages of WLED products include less heat-generation, less power consumption, longer lifetime, small size and rapid response speed. The average power consumption of WLED is about one-eight times that of incandescent lamps and one-half times that of fluorescent lamps. Accordingly, the average lifetime span of the WLED is more than 8,000 hours, which is 10 times higher than that of fluorescent lamps.
Currently, fabrication of WLED mainly includes two methods. The first method, generally, utilizes a plurality of LED chips, namely, red, green and blue LED chips are packaged together, and white light can be obtained by mixing those individual lights. The second method, generally, called a single LED chip. Various prearranged phosphors were excited by this single LED chip, and the white light was obtained by mixing the light of chip and phosphor. For example, yellow phosphors can be excited by a single blue LED chip to produce yellow light, then, the yellow light can be mixed with blue light emitted by the blue LED chip to produce white light. As the same way, the prearranged blue, green and red phosphors can be excited by an LED chip capable of emitting ultraviolet light to mix with and produce white light.
In FIG. 1, a cross section of a conventional white light emitting diode (WLED) is shown. In FIG. 1, an yttrium aluminum garnet (YAG) phosphor 4 absorbs blue light, which emitted from an LED chip 2 to produce yellow light. Then, yellow light emitted from the YAG phosphor 4, is mixed with blue light emitted from the LED chip 2 to obtain a white light. Fabrication of the YAG phosphor 4, however, is a complicated and a high temperature process. So that fabrication costs are expensive. Moreover, because the YAG phosphor 4 has a relatively huge diameter (micro-scaling) and is not easily dissolved in organic solvents, the YAG phosphor 4 is not easily and uniformly dispersed in an organic wrapping material. This phenomenon makes the uniformity of WLED less luminous. Furthermore, because red light is not provided, color rending index (CRI) of WLED is not good enough.
Additionally, in an ultraviolet LED chip type, in order to get white light, mixing red, green and blue phosphors is required. Accordingly, three kinds of phosphor need to be individual prepared at high temperature, the entire process is also time-consuming. Moreover, because luminous efficiency and decay rate of phosphors are different, fabrication and design of the WLED is complicated.
Thus, a light emitting diode device using a single phosphor and a method for fabricating thereof to improve color rending index and luminous uniformity is desired.